An automatic limit control (ALC), sometimes referred to as an automatic level control, circuit is used to dynamically adjust the level of a signal to avoid problems associated with clipped peaks, introduced noise, etc., due to large changes in the amplitude (commonly referred to as the “dynamic range”) of the signal. For example, loudspeakers in most notebook computers and cellular telephones have fairly limited dynamic ranges. The dynamic ranges of operating system alerts, such as “beeps,” and of conventional ring tones are sufficiently limited that a user may set a fairly high volume level, without risking distortion caused by clipping of peaks in the sound signals.
However, audio and video soundtracks typically have relatively wide dynamic ranges. Thus, users who play DVDs, etc. on their notebook computers or music on their cell phones may find themselves increasing the volume in order to be able to hear relatively quiet dialog or music, and decreasing the volume to avoid distortion during loud passages or sound effects.
Similarly, while using a public address system or a cell phone, a user may hold a microphone too close or too far from his or her mouth, speak in an uneven voice or change the distance to the microphone while he or she speaks. Such behaviors may cause the audio signal to become very loud, and possibly distorted, or become too soft, or fade in and out.
To avoid noise and distortion, and to reduce unnatural and annoying variations in audio signal levels, many systems use conditioning circuits to compress the dynamic range of the audio signals, such that faint signals are boosted, and loud signals are boosted by a lesser amount or are attenuated. Such signal conditioning improves the perceived quality or clarity of the signal.
The amount of compression in the conditioning circuit is typically expressed by a compression ratio r, which is a ratio of the change in amplitude of an input signal compared to the amplitude change of the amplitude of an output signal. At one extreme, a compression ratio of 1:1 produces no compression, i.e., the circuit provides fixed gain. At another extreme (such as a compression ratio of about 20:1 or more), the output is held relatively constant regardless of the input level. The latter situation is commonly referred to as an “infinite compression ratio” or “limiting.”
When a circuit's output signal level in decibels (dB) is plotted versus the input signal level in dB, the circuit's compression curve is generally a straight line whose slope (or the inverse of the slope) is equal to the compression ratio. If the compression ratio can be varied, the lines for different ratios generally intersect at a fixed reference point, called a rotation point. At the rotation point, the circuit's gain is the same for all of its available compression ratios.
At an infinite compression ratio, variations in the output audio signal that would otherwise result from clipped peaks or an improper use of a microphone are eliminated. However, the natural dynamic range of speech, music or other signal is also lost. In general, if the compression ratio is too large, the output signal amplitude is flat, and very low noise signals are amplified. On the other hand, if the compression ratio is too small, the microphone input problems remain, and the output may saturate and large signals may be clipped. The best perceived audio quality is usually achieved using a compression ratio of between about 2:1 and about 10:1.
A circuit may automatically respond to an output signal level to achieve a desired compression ratio. For example, a circuit may include an amplifier whose gain is automatically controlled in response to an output signal level. Such a circuit is disclosed in U.S. Pat. No. 5,631,968, titled “Signal Conditioning Circuit for Compressing Audio Signals” by Douglas R. Frey, et al., which is assigned to the assignee of the present invention. According to the above-referenced patent, a signal conditioning circuit may compress an audio signal by producing a gain control signal that is a function of the time-averaged audio signal and a fixed desired compression ratio, and amplifying the audio signal by an exponential function of the gain control signal.
Circuits, such as a dynamic range processor/dual VCA (voltage controlled amplifier) available from Analog Devices, Inc., Norwood Mass. as part number SSM2120, implement the signal conditioning circuit disclosed in the above-referenced patent. However, further improvements in the perceived quality of audio and other signals having large dynamic ranges are desirable.